Limitation On Two Way Use Of Cable Systems
TV cable modems operate in a hostile signal environment as the upstream cable channels are subject to generally uncontrollable ingress noise. Careful plant maintenance is needed to minimize the error causing effects of such noise. Many older cable systems are regarded as being unacceptable for reliable two-way transmission because of the cumulative noise level, and the lack of an easy way to determine the source of such noise necessary to correct its cause.
The Basic Problem
In the downstream direction, cable systems are similar in operation to a water distribution system with water flowing under pressure splitting into a myriad of smaller and smaller pipes to reach each house. In the reverse direction cable systems have been described as being similar in operation to a sewage system where each house contributes to a common flow gathered together by the system to reach the headend. Pollution from any single house can thus poison the common upstream channel for all subscribers in that accumulating path. Once polluted, discovering the source of that pollution is a highly manual labor intensive effort.
What is The Source of The Noise?
About 75% to 80%, or more, of the upstream noise is said to be generated within the houses themselves. Another 15% to 20% is attributable to the low-cost, flexible drop cables using inexpensive F-connectors that connect the houses to taps on the solid aluminum sheathed feeder cables passing the houses parallel to the street. While essentially all upstream noise originates from locations other than the feeder cable, a small amount of the noise is found to come from the feeder cable portion of the cable system. In practice, this small percentage of leakage into the feeder cable system is relatively easy to control. The FCC requires periodic leakage monitoring. To do so, a technician in a vehicle, having a radio receiver, listens to a cable frequency while driving along the street parallel to the feeder cable. Egress and ingress are two manifestations of the same phenomenon, and proportional to one another. Therefore, when easy-to-locate outgoing leakage is detected and corrected, the ingress at the same location, having been permitted by the same mechanism, is also cured.
The predominant noise sources are generally within the subscriber's house, where service access is often difficult. The in-house location of the noise is beyond the regulatory demarcation point of the cable franchise holding company, adding to the difficult task of locating and correcting sources of the noise within a subscriber's premises. The cause of the noise can be as varied as do-it-yourself cable wiring, light dimmers, universal electric motors with sparking brushes, poor electrical power wiring and inadequate grounding within the subscriber's premises. Thus, the major cause and location of noise in the short term is beyond the control of the cable company. This makes rapid detection and correction of the noise of the common polluted channel difficult. Further, even scheduling home visits to track down noise are increasingly difficult in a era when both spouses are away at work. This problem is compounded further in that the noise is often intermittent (e.g., present only when a vacuum cleaner is in use or a light fixture operated with a dimmer switch is on).
Need for More Robust Modulation
A heavy, but often unappreciated price, is already being paid by the cable industry in coping with limitations of the upstream channel. For example, the cable industry has tentatively chosen to use QPSK modulation in the upstream direction. QPSK modulation has only about two thirds of the data carrying capacity as the preferred standard downstream 64-QAM modulation. In other words, one-third of the potential upstream bandwidth capacity is the price paid to be able to better live with upstream noise.
Reduction In Number Of Houses Passed Per Channel
Another high price being paid is the necessity of reducing the number of houses connected to a common cable. Fiber optic cables must be extended further and further into the cable plant, to join the coaxial tails to correspondingly reduce the number of cable line amplifiers and houses connected on any single upstream channel, to correspondingly reduce the likelihood of ingress noise. For example, when the number of houses per upstream channel is reduced to 500 from 5,000, the probability of noise from any single house destroying the common channel is reduced by a factor of ten. While an excellent partial solution, the chance of encountering a noisy house that can bring down the entire system remains a significant risk factor.
Necessity for Dealing with Legacy Systems
The legacy cable systems in use today have all been designed to use the spectrum below TV Channel 2 (i.e., 5-42 MHz), for upstream transmission. In retrospect, this has been a poor choice of frequencies because of the ingress noise problem. What is needed is a viable approach to minimize ingress of noise to allow the effective use of this band for upstream transmission. The specific problem addressed by the inventors is the better commercial utilization of the 5-42 MHz band (particularly the 5 to 20 MHz portion of that band), so prone to short wave radio signal pickup, power line transients, harmonics and impulse noise.